For wireless networks, data is transmitted between base-stations and user equipment (UE) using radio frequency (RF) signals. The RF signals are generated by an RF signal source, and transmitted via an analog RF transmission link to an RF amplifier for amplification prior to transmission via a cell tower RF antenna. Traditionally, the analog RF transmission link comprises electrical elements such as RF analog cables
It is known that such an RF amplifier adds distortion to the RF signal being amplified. One known solution to this problem is to utilize a pre-distorter (typically a digital signal processor (DSP)) and a feedback path. The feedback path couples a portion of the output from the RF amplifier to the pre-distorter, which pre-distorts the signal being amplified to compensate for the distortion added by the RF amplifier.
For next generation RF networks, a separation between a base station (which processes the signal) and cell tower (which transmits and receives the RF signals over the air) has been proposed to allow for network virtualization which will allow for more flexible network architectures. For example, radio base band units (BBUs) will process the signals which are transmitted over the air by remote radio heads (RRHs). Such an architecture does not require the base-stations (e.g., the BBUs) to be co-located with the cell towers (e.g, RRHs). Such an architecture can allow for a single BBU to support multiple RRHs. Furthermore, such an architecture allows for more flexible cell tower placements, by reducing the signal processing required at the cell tower. Accordingly the signal processing occurs at the Base-Station. However this separation between base-station and cell tower requires the RF signals to be transmitted between a first location that processes the signal (such as a base-station) and a second location, such as a cell tower, that transmits and receives the RF signals over the air. Thus, the downlink (network to user) RF signal must be transmitted from a first to a second location. Similarly, the uplink (user to network) RF signal must be carried from the second to the first location.
To allow for the distance from the base-station to the cell tower to increase beyond the range of RF analog cables, RF analog cables are being replaced by optical waveguides, where such optical waveguides may comprise optical fibers. Accordingly Radio over Fiber (RoF) links are implemented to carry the RF signals from the Base Station to the cell tower.
However there is a need to address distortions/impairments introduced by such RoF links, in addition to the distortion impairments introduced by the RF amplifier as outlined above. This additional need increases as the separation between base-station and cell tower increases, thus increasing the length of the RoF links.